Fix flickering when updatig new tile (attempt 2)

[Emojigit]

Finally fixed the issue... *sigh* This PR is ready for review.

You can test it on my map server - you can see it updating by monitoring network requests.

[coveralls]

Pull Request Test Coverage Report for Build 9418311531

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 67.636%

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Totals
Change from base Build 9405508814:	0.0%
Covered Lines:	907
Relevant Lines:	1341

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💛 - Coveralls

[BuckarooBanzay]

so, do i get this right: this was caused by multiple calls to the onload function, right? 🤔

[Emojigit]

so, do i get this right: this was caused by multiple calls to the onload function, right? 🤔

Yeah, that's true. Leaflet changes the opacity every time onload gets called, i.e., a new tile is loaded.

One thing I forgot to implement: remove the onerror call too, and do such cleanup on onerror too